About 1.2 million metric tons of lactose each year result as waste product from the manufacture of cheese and in the whey-processing industry around the world. However, lactose as the principal carbohydrate constituent of milk has scarcely any economic importance as yet. One of the reasons for this is to be found in the lactose intolerance of some of the population. People with lactose intolerance are unable to utilize lactose and respond to consumption of lactose with symptoms of intolerance such as diarrhea. Only a relatively small part of the resulting lactose is utilized commercially, with lactose being employed for example as fermentation substrate, as filler or for the manufacture of dietary food products. Most of the resulting amounts of lactose are, however, disposed of through the manufacturers' waste-water treatment plants, possibly leading to disturbances of the ecological balance in water courses. However, since lactose is a raw material which is available in large quantities and at extremely reasonable cost, there is great interest in adding economic value to this carbohydrate. For this reason, various enzymatic cleavage and transformation reactions in which lactose is employed as starting material for manufacturing higher-value products in particular have recently been developed.
It is thus possible by oxidizing lactose to obtain lactobionic acid which is of great interest for a number of applications. The method employed to date for preparing lactobionic acid from lactose is an enzymatic one using the enzymes cellobiose dehydrogenase and hexose oxidase. The unsatisfactory conversion for example in the reaction catalyzed by cellobiose dehydrogenase can be increased by using the enzyme laccase which reoxidizes the redox mediators which are reduced in the reaction. Because of its excellent ability to form metal chelates, lactobionic acid is employed inter alia in the so-called Wisconsin transplantation solution, because lactobionic acid is able to reduce the oxidative damage, caused by metal ions, during storage of organs for transplantation. Lactobionic acid can likewise be employed as biodegradable cobuilder in washing powder, which may comprise up to 40% lactobionic acid. Because of the mild sweet-sour taste of lactobionic acid there are further possible applications in food technology.
There is also great potential for use of other aldonic acids or oligosaccharide aldonic acids in the pharmaceutical industry, the manufacture of cosmetics and in food technology. Aldonic acids are currently prepared mainly by microbial or enzymatic conversion from the corresponding mono- or oligosaccharides. Thus, for example, glucose can be converted into gluconic acid by using Acetobacter methanolicus. The enzymatic preparation of aldonic acids is, however, generally characterized by a relatively low productivity and is also not without problems for environmental protection reasons. There is thus a great interest in alternative oxidation methods leading to distinctly less environmental pollution, where the carbohydrate to be oxidized, for example a monosaccharide, is oxidized to the corresponding aldonic acid using a heterogeneous catalyst.
Heterogeneous catalysis of an oxidation reaction normally takes place in a three-phase reactor, with the solid catalyst, usually in powder form, being suspended in a liquid phase comprising the compound to be oxidized, and oxygen being bubbled through the liquid phase during the reaction. Although catalytic oxidation has some considerable advantages by comparison with the enzymatic reaction, especially in relation to considerably less environmental pollution, it does have a decisive disadvantage. When metals are used, activation of dioxygen may lead to free-radical reactions which may, especially in the case of polyfunctional molecules, distinctly reduce the selectivity of the conversion (Sheldon and Kochi, “Metal Catalyzed Oxidations of Organic Compounds”, 1981, Academic Press, New York).
The use of supported palladium and platinum catalysts for oxidizing glucose has been investigated most thoroughly to date. It has emerged from this that on use of these catalysts there are considerable limitations on the catalytic conversion of glucose into gluconic acid because of the low selectivity and reaction rate. Moreover, deactivation of both types of catalyst is relatively rapid. This deactivation is evidently derived either from blocking of the catalyst surface owing to adsorption of molecules or from poisoning effects caused by dioxygen (Van Dam, Kieboom and Van Bekkum, Appl. Catal., 33 (1990), 187). Some of the factors which limit the catalytic conversion of glucose to gluconic acid can be distinctly improved by introducing promoters such as bismuth or lead. Besides an improvement in the catalyst life, in particular the reaction selectivity and the reaction rate are increased thereby (Fiege and Wedemeyer, Angew. Chem., 93 (1981), 812; Wenkin et al., Appl. Catal. A: General, 148 (1996), 181).
However, there is controversy about Pd and Bi because of possible leaching of these toxicologically objectionable substances. Slightly alkaline conditions are necessary to increase the reaction rate and to prevent catalyst deactivation. However, side reactions which reduce gluconate production occur under such conditions. It is likewise disadvantageous that gluconate is produced instead of free gluconic acid on use of bases (Biella, Prati and Rossi, Journal of Catalysis, 206 (2002), 242-247).
For these reasons, the fermentation process is still preferred for industrial production of gluconic acid, despite the problems arising with this method, for example the heavy contamination of waste water and the not inconsiderable formation of by-products. This is why there is a need to develop novel types of catalysts which make catalytic oxidation of carbohydrates possible for preparing aldonic acids using dioxygen as oxidizing agent, and having a long useful life besides high activity and selectivity.
Supported gold catalysts have been employed to date in particular for oxidizing CO or propene in the gas phase and for selective hydrogenations. Biella et al., Journal of Catalysis, 206 (2002) 242-247, describe the use of a carbon-supported gold catalyst for selective oxidation of D-glucose to D-gluconic acid in the liquid phase. Comparison between the carbon-supported gold catalyst and conventional palladium and platinum catalysts shows that the gold catalyst is superior in several respects to both palladium and platinum catalysts. In particular, compared with palladium and platinum catalysts, the gold catalyst used is substantially more stable to deactivation. A further advantage of the gold catalyst used is that no external pH monitoring thereof is necessary in the glucose conversion. The carbon-supported gold catalysts used do, however, have a considerable disadvantage. On the one hand, the leaching of gold out of the catalyst increases as the pH falls. On the other hand, growth of the gold particles is promoted as the pH rises. Both lead to a decrease in the catalyst activity. The increasing dissolution of gold particles as the pH rises is associated with an enlargement of the gold particles. This is probably attributable to the fact that small gold particles dissolve and then the gold is deposited on larger gold particles, resulting in a reduction of Au(I,III) particles.